Other measures of vegetation like the Normalized Difference Vegetation
Index use the ‘greenness’ of leaves as indicator of chlorophyll
content. Unfortunately, chlorophyll content adapts slowly to stress (it
can take weeks for leaves to lose their green color) and is therefore
not as useful as a proxy for direct GPP. As photosynthesis is the core
mechanism that drives both carbon assimilation and fluorescence,
fluorescence can be exploited to serve as a more direct proxy for GPP.
This allows for eyes on the short-timescale variability in terrestrial
vegetation activity. During photosynthesis, sunlight is absorbed by
chlorophyll pigments in leaves and converted into chemical energy.
Approximately 80% of the harvested energy is used for photosynthesis.
Most of the rest is dissipated non-radiatively as heat and a small
fraction (~1%) is emitted at longer wavelengths as fluorescence. This
SIF has a spectrally smooth signature and peaks at 737 nm.

Space based spectrometers observing this spectral region with sufficient
spectral resolution and signal to noise ratio are able to detect the SIF
signatures. Following
recent advancements in space borne observatories, methods have been
developed to disentangle the small SIF signal from radiance spectra of
the GOME-2 instruments. The retrieval produces monthly averaged SIF maps
on a 0.5x0.5 degree grid for the GOME-2A instrument.